1. Field of the Invention
The present invention relates to a small-size wide angle lens and a camera including the small-size wide angle lens. In particular, the present invention relates to a small-size wide angle lens appropriate for a digital camera and a camera including the small-size wide angle lens.
2. Description of the Related Art
Generally, a lens for a compact digital camera needs to have a short back focus to reduce the size of the camera, on which the lens is mounted, when the camera is carried by a user. Further, it is important that the diameter of the lens is small. As such a lens for a digital camera, lenses disclosed for example in Japanese Unexamined Patent Publication No. 2010-113248 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2008-040033 (Patent Document 2) are known. Patent Document 1 discloses a lens system consisting of a negative lens group and a positive lens group, which are arranged in this order from the object side. Further, the F-number of the lens system is 2, and the 35 mm silver halide film camera equivalent focal length of the lens system is 28 mm. Meanwhile, Patent Document 2 discloses a lens system substantially consisting of a negative lens group and a positive lens group, which are arranged in this order from the object side, and the F-number of the lens system is approximately 4.
The lens systems disclosed in Patent Documents 1 and 2 are single focal lens systems, in which focal lengths are fixed. As a zoom lens system having variable focal length, and the size of which has been tried to be reduced, a lens system is disclosed in Japanese Unexamined Patent Publication No. 8 (1996)-179215 (Patent Document 3). The lens system disclosed in Patent Document 3 consists of a positive lens group, a positive lens group and a negative lens group, which are arranged in this order from the object side. Further, each lens group includes a negative lens and a positive lens, and the F-number of the lens system at a wide angle end is 5.7.
Generally, it is desirable that rays of light enter an imaging device arranged at an image plane of a lens for a digital camera at an angle close to 90 degree to the image plane, which means that the rays of light entering the imaging device are perpendicular to the image plane. Therefore, it is desirable that a stop is located at a distant position away from the imaging device in the lens system. However, in such a lens system, a distance in the direction of an optical axis from a most-object-side surface in the lens system to the imaging device tends to become long, and that has been an obstacle for reducing the size of a camera.
Conventionally, the size of a camera when the camera is carried by a user was reduced by adopting a collapsible mount method. In the collapsible mount method, a lens is collapsed into a camera body when the camera is not used for photography, and the lens is slid out from the camera body to perform photography. However, in the collapsible mount method, time is needed to slide the lens out from a collapsed position to make the camera usable. Further, operations for sliding out or collapsing the lens consume power of a battery of the camera. Therefore, there has been a demand for a small-size lens system that can constitute a camera that is compact both in a carried state and in a photography state without using the collapsible mount method. At the same time, there has been a demand for a high performance lens system that has a sufficiently wide angle to satisfy a user's demand in recent years, and which has a small F-number.